In many areas of the world the availability of water to maintain the natural growth of plants is either insufficient or unreliable, especially if the plants are not native to the area. For centuries this problem has been overcome by the development of irrigation systems where water is transferred from a local available source such as a river, dam or bore and used to artificially irrigate the plants.
The twentieth century has seen the further development of irrigation systems to the level of total computerised automation. This has resulted in many areas of the world having large tracts of previously unusable arid land that are now capable of producing crops of all types.
A typical irrigation system comprises of a network of underground pipes along which water is pumped. Selected valves at strategic points on this network, when activated, release water to local distribution points such as sprinklers or drippers. The method of activating these valves may vary, but typically they would be triggered by electrical, mechanical, hydraulic or manual means.
The most common electrical device is an electro-mechanical solenoid. An activating current causes the solenoid to move a spring-loaded plunger, allowing the valve to open due to the water pressure in the irrigation pipes. When this current is either removed or possibly reversed, the plunger returns to its original state thus allowing the valve to close.
The solenoids are activated, either directly or remotely, by an electrical or electronic control systems such as irrigation controllers, programmable logic controllers (PLC's) or even manual switches.
The most common form of irrigation solenoid is activated on application of a voltage of 24 volts AC. Other solenoids activate on a range of different voltages from 6 to 48 volts, either being AC or DC. In order to minimise power consumption, latching solenoids are available which enable on the receipt of a voltage pulse of one polarity and disable when a voltage pulse of the reverse polarity is received.
The typical means of transferring the current required to activate these solenoids is a pair of cables running for distances of up to two kilometers from the controlling system. The limitations on this distance are dependent on the resistance of the cable such that sufficient power is available to activate the solenoid for the required time.
Commercial irrigation sites such as farms, parks or golf courses can cover large areas, consequently the length of cabling required to service all the solenoids may run to many kilometers. Currently there are two main techniques in use to distribute power to the solenoids, referred to as ‘Direct Connection’ and ‘Two-Wire’. A brief description of these techniques follows.
Direct connection is the older or more traditional method, which is to supply power directly from an activating relay (or similar electronic device) within a control system by a directly connected pair of cables. It should be noted that the word ‘pair’ only refers to the connection point at the solenoid, as the typical wiring layout of such an installation is normally a matrix of single cables with the ‘pairs’ only occurring at the required solenoid junction locations.
Two-wire systems provide both power and activating commands along a single network. This network is generally consists of a true ‘pair’ of cables and each solenoid within the network is activated by a corresponding decoder connected between it and the network. A master irrigation controller powers and issues commands to the decoders via the pairs of cables. The format of the command communications depends on the manufacturer's preference. Many existing systems utilise tone or DTMF (Telephone-type tones) signals superimposed on the powering voltage. Normally (and preferably) the network is wired in a ‘point to point’ configuration between the master irrigation controller and the decoders.
Most control systems activate solenoids by applying a 24v AC 50 Hz RMS power signal to the solenoid. Although this technique appears both obvious and simple, a number of problems and limitations do occur.
A typical solenoid used requires around 3 watts at 24v AC to hold in, resulting in a holding current of around 300 mA. When the solenoid is activated, the inrush current can be double (or more) the holding current. The inrush current must be maintained until the plunger has fully seated.
One example of inrush current increases in duration is where a solenoid plunger is clogged with sediment or sand. On activation, if the force of the solenoid is not sufficient to move the clogged plunger, the plunger would vibrate violently at the waveform frequency and could take a number of seconds to activate. In this case the instantaneous inrush current would have to be maintained for far longer periods before the solenoid would be fully activated. If this solenoid was being activated some distance from the voltage source (the irrigation controller) or if other solenoids were also being activated which used common cabling runs, the resistance of the wire could cause the following scenarios to occur:                The solenoid would not activate.        The voltage drop and solenoid-induced interference at the decoder could be sufficient to cause the decoder electronics to reset, fail, or run unreliably.        If the irrigation controller is equipped with current sensing, it could shut down the section being irrigated and skip to the next section.        The current drawn (under worst cases) could cause a fuse to blow or trip at the irrigation controller. In this case irrigation could be suspended or cancelled.        
Disregarding back-EMF voltages and other considerations, it may generally be assumed that when a solenoid is activated by an AC sinusoidal voltage the maximum amount of current flow occurs at the 90 and 270 degree points of the waveform, with the zero cross (no current drawn) occurring at the 0 and 180 degree points.
As more solenoids are activated simultaneously, the current draw will consequently increase. If two solenoids are activated with similar characteristics then the current draw will almost double, The difference will depend on the resistance and length of the supplying cable. Currently most two-wire systems start to become unreliable when operating multiple solenoids over distances exceeding one or two kilometers (utilising standard irrigation cabling). Some manufacturers overcome this problem by specifying thicker or custom manufactured cabling, which greatly increases the cost of the installation.